This invention relates to solids-liquid separating centrifuges which have a rotary bowl, a rotary conveyor mounted coaxially therein and means for rotating the bowl and conveyor about their common axis in the same direction at a differential speed. More particularly, the invention relates to solids discharge mechanism for such a centrifuge and to the provision of such mechanism which is particularly useful where hydraulic assist is used in conveying difficult to convey solids to a solids outlet.
Centrifuges of the type concerned normally discharge the solids through a frusto-conical interior of an end portion of the bowl, defining an outlet at its smaller end which may be a series of apertures in its wall. The solids discharge mechanism comprises this bowl end portion and a bladed hub on the conveyor extending therein to convey the solids to the outlet. The bowl interior preceding the larger end of its frusto-conical end portion may itself be frusto-conical, a continuation of its said end portion, or may, more usually, be cylindrical. The conveyor preceding the hub portion may be a cylindrical helically bladed hub or cage.
In order to convey the solids through the frusto-conical interior of the bowl end portion, some of the prior art has proposed utilizing a frusto-conical hub having a smaller cone angle than the passage in which it fits, as in U.S. Pat. Nos. 775,320 and 3,379,368. In such a construction the passage between the hub and the bowl end interior diminishes in width and cross-sectional area, and therefore in capacity, to its smaller end, and thus has a tendency to plug, particularly if the hub is provided with a conveyor blade helix of diminishing pitch (distance between centers of successive complete turns) toward the outlet, as in U.S. Pat. No. 775,320 aforesaid. U.S. Pat. No. 3,379,368 seeks to avoid such plugging by providing a blade helix of increasing pitch toward the outlet.
U.S. Pat. No. 3,143,504 forms the inner, frusto-conical hub at a larger cone angle than the conically shaped bowl end within which it fits, so that the passage between them diverges to an outlet end which, as shown, has a larger transverse cross-sectional area than the inlet. The blade has an increasing pitch toward the discharge end, as is shown by increasing distance between the blade turns.
A more recent commercial centrifuge of the prior art uses an inner cone with a larger cone angle than that of the bowl end cone within which it fits, but selects the cone angles such that the progressive increase in width of the passage between the cones from inlet to outlet substantially offsets the progressive decrease in diameter, so that the cross-sectional area of the passage at its outlet end approximately equals the cross-sectional area at the inlet. The cross-sectional area of the passage is accordingly substantially the same throughout its length (there is a small enlargement in the middle unless one of the cones is formed to a curved generatrix to offset it). The blade helix has a constant pitch. This mechanism operates satisfactorily with readily conveyable solids to produce acceptably dry solids outputs or "cakes". However, in treating sludges containing a large proportion of soft, slimy solids which are essentially non-conveyable by the blade helix, "hydraulic assist" of a deep slurry pool in the bowl, having its inner surface nearly as close or even closer to the centrifuge axis than the solids outlet, is needed to apply hydraulic pressure at the inlet sufficient to force such solids through the helix. With such sludges and hydraulic assist, difficulty has been experienced in producing a discharge cake of acceptable dryness.